In German patent document DE 26 30 446 C2 and U.S. Pat. No. 4,083,530, an elongated support is disclosed in which a pair of telescoping members, namely, a support tube and a hollow bar are actuated by a head formed at the mouth of the tube from which the bar emerges.
That head comprises a drive roller which can be rotated by an external fork, e.g. via a wrench, crank, key or the like, and is wedged between a running surface of the bar and an inclined surface of the head so that, as the roller is rotated, the bar can be extended from the tube.
Under load, however, the roller is jammed between these surfaces in a wedging self-locking action which locks the bar and the tube against contraction.
The roller can be toothed along its surface engageable with the running surface of the bar so that the teeth can bite into this latter surface.
Furthermore, the actuating head can be provided with a guide element which has slots extending generally transversely to the running surface of the bar and the longitudinal dimension of the support and in which the ends of the roller are engaged so that, upon pivoting of this element on the head, the roller can be urged out of its wedged position to release the lock and enable telescoping of the bar relative to the tube.
The guide element can be swingable against the force of a prestressed spring and can have a fork shape. An actuator for the fork, e.g. a handle, can extend substantially parallel to the longitudinal dimension of the support and, under the action of the prestressed spring, can be spaced from the support tube. By manually overcoming the spring force, a user can press the lever or handle closer to the support tube to release the roller.
When, however, the hand of the operator releases the lever or handle, the spring action will restore the handle to its rest position in which the spring presses the roller into its wedging position between the surfaces of the actuating head and the support bar.
Thus, as described in DE 26 30 446 and U.S. Pat. No. 4,083,530, the lever-like handle serves, upon displacement toward the support tube, to bring the drive roller out of engagement with the corresponding running surface of the bar to allow the support to be manually extended or contacted by shoving the bar out of or into the outer tube.
When the elongatable support is to be extended under load, the drive roller can be rotated with a key, wrench or other tools engageable with, for example, a polygonal formation on the drive roller. In this case, the lever-like handle is not actuated and often cannot be since the force applied to the drive roller when the support is under load can exclude the lifting of the drive roller out of its wedged position by a manual swinging of the handle.
However, there are some cases in which the load cannot preclude the swinging of the handle out of position. For example, when the load applied to this support is small, there is a danger that accidental contact with the lever-like handle may unlock the support and prevent the load from being supported, the support then contracting involuntarily. It is also possible, under certain conditions, for gravitational force on the lever-like handle to overcome the prestressed spring action and the effect of a light load and release the support from its locked condition.